The primary object of this invention is to provide a system for identifying an object, animal or person consisting essentially of two units, one being a passive integrated transponder (PIT) which is carried by or embedded in the thing or animal to be identified and which responds to interrogation with an identifying code, and the other unit being an interrogator-reader separate from the PIT.
Heretofore, in identification device systems, there is usually found some sort of an exciter which is associated with an interrogator which feeds a combined alternating current clock signal and power supply signal to a responder device via an inductive coupling. The responder may be implanted in an animal or on something whose identity is to be ascertained, such as a freight car. The responder produces a coded identification signal which is fed back through the inductive coupling to a detector and demodulator which produces an output signal characteristic of the particular animal or thing that is being identified.
Many coded data transmission systems have been used. In some the responder entails a resonant circuit the resonant frequency of which varies according to the encoded signal peculiar to the animal or thing being identified. In Kaplan et al. U.S. Pat. No. 3,689,885, coded information was returned from a responder to an interrogator in the form of spaced bursts of alternating current of a fixed frequency. In Beigel U.S. Pat. No. 4,333,072, for example, the responder or tag circuit produces a signal by varying the load across the inductor responsive to the encoded signal characteristic of the animal or thing being identified. All of these systems were subject to certain drawbacks: the resonant circuit systems particularly being subject to variations in resonance frequency produced by, for example, changes in temperature which, in some instances, could produce spurious variations in frequency which were difficult if not impossible to differentiate from variations produced by the encoding device. Systems wherein the code signal was represented by variations in amplitude of an oscillating circuit have the drawback that such variations in amplitude are difficult to accurately detect.